Firstly, increasing a demon's defense with the "Stone Skin" perk... instead of what? Perhaps instead of attack or dark magic? This is more in the realm of exotic choices. It's much more useful to focus on luck, which improves gate opening and attack power, or leadership, which widens the gate and consequently increases already considerable initiative. In a prolonged battle, a well-developed gate opening skill can be more useful than defense, and education won't hurt either.
Secondly, regarding archers. In that case, mass confusion is always cast first, and even if the enemy hero has mana left (which is not a given) and spends their turn removing the spell or using antimagic on one stack, they will still be at a disadvantage. Moreover, mass perks cost half of the hero's turn...
Each race has its own strengths, and by leveraging these strengths, the race achieves success. I believe the task of optimal development is to maximize your advantages so that the enemy doesn't have time to exploit your weaknesses.
Powerful defense will still not become a strength for demons; it will remain a weakness, albeit a less noticeable one.
The demon's strength lies in attack and initiative, and of course, in summoning reinforcements. And if the demon's goal is to win, not to hold the battlefield for as long as possible, then it should attack as often and as powerfully as possible.